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I've got a fairly significant judder, which isn't consistent. It's on light to medium braking, but braking hard is fine... making me think it's not warped discs. Also seemed worse when going uphill... making me think its a steering geometry thing.

Front brakes are arp calipers and discs (done by previous owner),

I've seen some posts on here pointing to issues caused by front suspension arm bushes, so I got it on the ramp today to take a look. This is the only slightly dodgy looking g Bush on the rhs lower arm, what do you all think, could it just be this?

Sorry that should be "ap racing calipers" (got engine builds and head studs on the brain at the mo)

Although getting a disc to "warp" is extremely rare (in fact it's almost impossible with AP brakes on a road car) you get the same symptoms your describing from uneven Pad deposits which the AP setup are known for as they are more track focused (lots of rapid high heat events over a short duration) rather than road focused (lots of light braking do over a long time).

So basically before spending any money or changing anything go through a "Pad Rebedding" procedure first just to confirm it's not this. I also have AP Radi-Cal Calipers and 410mm AP discs and this is what I do (every brake manufacturer has a different version, mine is based on AP's more track focused discs rather than "road" discs but be aware everyone thinks differently about this) -

First find a long deserted road for this!!! Then brake extremely hard from 70-80mph (emergency stop but without activating the ABS) down to 10mph but do not come to a stop and then immediately accelerate back to 70-80mph and then immediately brake hard down to 10mph again - Repeat this 7-10 times back to back without stopping and then, after the last time you need to cool the brakes down so keep driving touch the brakes as little as possible and if you do brake be as light as possible. If you do have to stop then at no point should you keep your foot on the brake, instead put the car into Park if waiting at lights etc. After 30 minutes of "cooling" the Pad Rebedding is complete and the disc face should have a dark black/bluey shine to it and not be Silver any more.

This dark colour is the Pad material which has been transferred to the Disc. As AP Racing discs are more track based, the material used tends to be "harder" than everyday road discs which means this thin pad layer is more easily worn away by everyday normal braking and so your discs will slowly turn shiny Silver again over time as the material goes and the "judder" will return as various bits of the pad material are worn away unevenly. If I drove my car like an A6 for around 2 months I would have to rebed the pads again but, if around once a week I did 2-3 "hard" stops it meant I didn't have to do a full rebed - Ah the joys of Track orientated AP Discs!

Once the judder returns just rebed the pads again.

These pics show what mine looked like Before/After rebedding but they're not great at showing the difference TBH -

Before - Nice and shiny Silver -



After - Darker colour, not as bright Silver any more -



A more extreme example is my old AP 390mm discs and my new 410mm discs - Shiny Silver vs Darker black/blue silver - The darker colour is the pad transfer, the more silvery looking strip on the smaller discs is where "everyday" driving was wearing that darker pad material section down.

Thanks, I've tried this a couple of times now and it's slightly better, but the judder is still there when braking from more than about 60mph. How violent is it normally when you've had this? As mine is to the point where the steering wheel shakes.

If rebedding didn't work after the first attempt (7-10 hard brakes) then it's definitely not uneven pad transfer so, now you've done the Free diagnostics to confirm whether it's still the same, and it is, you can be sure it's a knackered arm bushing which is going to cost money to fix.

Personally I'm a fan of the Meyle HD suspension arm kits but, as they replace all the arms, it's far more expensive than replacing a single arm (still cheaper than Audi OEM) but Lemforder are pretty cheap but still good quality.